Haas is through to the semis in Melbourne for the third time

Haas, seeded 12th, won 6-3 2-6 1-6 6-1 7-5 in three hours 19 minutes and will play second seed Rafael Nadal or 10th seed Fernando Gonzalez in the semis.

"That's one of my best wins, being down a break in the fifth," said Haas, who saved a match point at 4-5.

"I don't know how I did it, I was just hanging tough. I'm so happy right now."

Davydenko looked on course for victory when he moved two-sets-to-one in front but Haas swept through the fourth to set up a tense decider.

It's one of my biggest matches I've won, for sure Tommy Haas

The decisive moment seemed to have arrived in game 10 when Davydenko had a match point on Haas's serve, but he buried a forehand into the net.

Haas then played a superb game to break but there was more drama on the German's first match point in game 12.

Davydenko's return was called out but he successfully challenged thanks to Hawk-Eye, before Haas put his vociferous supporters out of their misery by taking his second match point.

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The 28-year-old German will now play in his third Australian Open semi-final, having lost to Yevgeny Kafelnikov in 1999 and Marat Safin in 2002.

"It's one of my biggest matches I've won, for sure," said Haas.

"In the quarter-finals, to save a match point, the number three player in the world, I've never beaten him before, coming back from a break down, match-point down.

"Another reason why I'm really happy is now I finally beat a Russian in a big match here. I lost to (Yevgeny) Kafelnikov and to Marat (Safin) in the semis here.

"If I would have lost the quarter-final here against another Russian, it would have been really like never going back there again."